Help understanding a spell

[RLFHOG3]

Someone help me understand what this spell does:

Link (http://dndtools.eu/spells/players-handbook-ii--80/energy-vulnerability--2911/)

Energy Vulnerability, from PHBII, is a pretty cool spell and i like it but... the wording of the spell confuses me. Because on one end of the spell, it says that a will save negates the effects of the spell.
but on the other end....
"When you cast this spell, specify an energy type (acid, cold, electricity, fire, or sonic).
The affected creatures (in a 10 ft radius burst) gain vulnerability to that energy type (they take +50% damage from that energy, even on a successful save)."

Why even bother having a save with this spell? What does the save even negate?

[darkbuu_1]

I believe the first save it mentions is to do with this spell taking effect and the "even on a successful save" is referring to a character saving against an relevant elemental attack, such as someone successfully saving against burning hands while having fire vulnerability. However I'm not sure if they would (Damage*1.5)/2 or (Damage/2)*1.5.

[Lo77o]

It is part of what happens when you have a vulnerability... You take 50% extra damage against that energy type. So if you have fire vulnerability, you would take 50% more damage from a fireball, even if you managed to make your reflex save for half.

It does not refer to the save of the "Energy Vulnerability" spell, its simply clarifies how vulnerability works against an energy type.

[Forrestfire]

[The affected creatures gain vulnerability to that energy type] [(they take +50% damage from that energy, even on a successful save).]

These are separate clauses. A will save negates the energy vulnerability spell as stated. The text inside the parentheses is just a reminder of how Vulnerability works.

[RLFHOG3]

OK. That clears things up nicely. Thanks a bunch. I was having a hard time trying to understand what in the heck that was supposed to mean.